water, the
water will creep a little way up the finger, though you may not stop to
examine it. I have here a substance which is rather porous--a column of
salt--and I will pour into the plate at the bottom, not water, as it
appears, but a saturated solution of salt which cannot absorb more; so
that the action which you see will not be due to its dissolving anything.
We may consider the plate to be the candle, and the salt the wick, and
this solution the melted tallow. (I have coloured the fluid, that you may
see the action better.) You observe that, now I pour in the fluid, it
rises and gradually creeps up the salt higher and higher; and provided the
column does not tumble over, it will go to the top.

[Illustration: Fig. 1.]

If this blue solution were combustible, and we were to place a wick at the
top of the salt, it would burn as it entered into the wick. It is a most
curious thing to see this kind of action taking place, and to observe how
singular some of the circumstances are about it. When you wash your hands,
you take a towel to wipe off the water; and it is by that kind of wetting,
or that kind of attraction which makes the towel become wet with water,
that the wick is made wet with the tallow. I have known some careless boys
and girls (indeed, I have known it happen to careful people as well) who,
having washed their hands and wiped them with a towel, have thrown the
towel over the side of the basin, and before long it has drawn all the
water out of the basin and conveyed it to the floor, because it happened
to be thrown over the side in such a way as to serve the purpose of a
syphon.[5] That you may the better see the way in which the substances act
one upon another, I have here a vessel made of wire gauze filled with
water, and you may compare it in its action to the cotton in one respect,
or to a piece of calico in the other. In fact, wicks are sometimes made of
a kind of wire gauze. You will observe that this vessel is a porous thing;
for if I pour a little water on to the top, it will run out at the bottom.
You would be puzzled for a good while if I asked you what the state of
this vessel is, what is inside it, and why it is there? The vessel is full
of water, and yet you see the water goes in and runs out as if it were
empty. In order to prove this to you, I have only to empty it. The reason
is this,--the wire, being once wetted, remains wet; the meshes are so
small that the fluid is attracted so strongly from the one side to the
other, as to remain in the vessel although it is porous. In like manner
the particles of melted tallow ascend the cotton and get to the top; other
particles then follow by their mutual attraction for each other, and as
they reach the flame they are gradually burned.

Here is another application of the same principle. You see this bit of
cane. I have seen boys about the streets, who are very anxious to appear
like men, take a piece of cane, and light it and smoke it, as an imitation
of a cigar. They are enabled to do so by the permeability of the cane in
one direction, and by its capillarity. If I place this piece of cane on a
plate containing some camphin (which is very much like paraffin in its
general character), exactly in the same manner as the blue fluid rose
through the salt will this fluid rise through the piece of cane. There
being no pores at the side, the fluid cannot go in that direction, but
must pass through its length. Already the fluid is at the top of the cane:
now I can light it and make it serve as a candle. The fluid has risen by
the capillary attraction of the piece of cane, just as it does through the
cotton in the candle.

Now, the only reason why the candle does not burn all down the side of the
wick is, that the melted tallow extinguishes the flame. You know that a
candle, if turned upside down, so as to allow the fuel to run upon the
wick, will be put out. The reason is, that the flame has not had